Water heater control system



y 1945- c. M. OSTERHELD WATER HEATER CONTROL SYSTEM 2 Sheets-Sheet 1 Filed Nov. 25, 1943 7 3 2 m M w 3 n 7\, w 9 n 3 "Q2 9 I l-I\ 9 7 w ak Cum? ATTORNEY May 8, 1945. c. M. OSTERHELD I WATER HEATER CONTROL SYSTEM 2'Sheets-Sheet 2- Filed Nov. 25. 1943 624M 9AM NOON -96 9 INVENTOR.

ATTORNEY k i Osrawa CLAR Patented May 8, 1945 WATER HEATER CONTROL SYSTEM Clark M. Oster'held, Stoughton, Wla, assignor to MoGraw Electric Company, Elgin, Ill., acorporation of Delaware I Application November 25, 1943, Serial No. 511,731

My invention relates to electric heating sys- I tems and particularly to electric water heater control systems.

This invention relates to electrically heated storage water heaters especially adapted for domestic use where the general demand for electric current from the power plant is heavy or onpeak, as usually stated, for a portion, or portions of the twenty-four hour day, and is relatively light or, off-peak" for another, or other portions of the twenty-four hour day.

In some parts of the country timer controlled switches are used to arbitrarily limit the periods during which certain apparatus, such as electric water heaters, can be connected to the supply line to receive energizing current and, as a rule, the period is arbitrarily made to start when experience has taught that a decline in the demand of current may be expected regularly and to end when or slightly before when like experience has taughtthat an increase more or less sharp in the demand, may be expected regularly.

One of these periods usually runs from about ten p. m. to six 9.. m. and includes a period after midnight in which there is a minimum demand and current can be sold cheap in order to measurably even up the load on the generating equipment. Hence, it is a great advantage to heat water for the day's use during this period after midnight, and store it up for withdrawal during the on-peak hours. But as a practical measure that alone does not meet all requirements, form the nature or. things some extraordinary with drawal of hot water will be made each day by a few customers, not always the same. outof many taking current from the same source, and a customer who has an emergency during the onpea hours occasionally should be able to replenish his hot water supply without waiting until after midnight.

As longas the .use of current outside of the after midnight period for heating water is conv lined to a 'few out of many, the scheme is not upset. Cutting in at ten p. in. after an emergency may be made the privilege of all with the appropriate storage tanks. But those who regularly require water heating at or before ten p. m.- may be put to the necessity of increasing their storage -tank, v

In other parts of the country time switches are not used because the cost is prohibitive or they are otherwise objectionable. But there is still the Claims. (cram-s9) maximum, or at a minimum, with a few variations, out of many users.

The principal object of the invention is to provide a control system that will automatically furnish immediate heating when the withdrawal of hot water has been a large part of the supply and, hence, there is a large portion of cold water in the storage tank to be heated or will automatically delay heating when the withdrawal has been but a relatively small part of the supply, and will also automatically make thes variations with appropriate relation to the on-peak and oil-peak" periods and thus enable the necessary current for most of the heating to be supplied during the hourswhen the other demands for current are below the peak or at a minimum.

Generally speaking this is accomplished by making the actual water heating depend on the joint and cooperative action of three control means;

First, afirst switch for the heater operatively associated with a, thermo-sensitive element sub- Ject to heat exchange with tank water'in the lower portion of the storage tank, which always serves to open that switch when the water in the a lower portion of the tank'is hot and to close it, or

allow it to close when the water in the lower portion of the tank is cold;

Second, a second switch for the heater operatively associatedwith a thermostatic element sub- .iect to heat exchange with water in another portion of the tank and preferably somewhere in the medial portion, and

Third, a-control device subject to some influence other than the temperature of the water in the tank, such as auxiliary heating, for modifying the action of the second switch and insuring that the heater circuit will not be made operative until a time appropriate to the amount of coldwater to I be heated for replenishing the normal supply of hot water. I In what is now considered the best practice, the

.auxiliary control is effected through a small electric heater, well insulated from adjacent parts and cooperating with the -thermo-sensitive element subject to tank water temperature between suitable heating period.

need to confine the main use of current for such purposes as domestic water heating, to periods in which the demand for other uses is. below the the ends of the tank to regulate and provide the In those areas where the power company pre fersto use time cut-oil switches to arbitrarily limit the period for heating water for domestic use, the thre epart control will become operative as soon as th time switch makes thecurrent from the line available and will continue in control-to;

furnish the heat to replenish-the supply of hot water at or just safely before the end of the period in which the current is made available.

. The three-part control device may be that shown in the present application or it may take other forms operative for the same purpose.

The system disclosed and claimed in the present application has a distinct advantage over the system includinga time clock in that an individual heater is given current according to its'needs.

, The power company is still getting off-peak heater operation for this load as a group of, say, a hundred tank heater installations, while the individual variations, say one, two or five, from offpeak operation, is not enough to be objectionable. From the customer's standpoint, if he were subject to time clock control to effect energization of the heater only during off-peak hours, he would not be able to have special capacity on the few occasions when his special need demands it..

An object of my invention is to provide a relatively simple system for controlling the energization of an electric heater for a domestic hot water tank subject to withdrawals of varying quantities of hot water during a twenty-four hour day.

Another object of my invention is to provide a control system for a domestic hot water tank subject to withdrawals of hot water during the day that shall be effective to cause immediate energization of the electric heating means in case of withdrawals of relatively large quantities of hot and dwellings.

ings, I have there disclosed a hot water tank H which for illustrative purposes is shown as of the usual domestic hot water type used in homes material l3 therearound, which heat-insulating material is held in place by an outer casing 15. I provide a cold water inlet pipe i! at the bottom end of the tank. 'I provide also an upper hot water outlet pipe l9, all in a manner now well known in the art.

I provide further an electric heater 2| which may be of any suitable or desired kind, though I have shown it as being mounted on and around the outside of the tank I l and positioned in a tunnel member 23.

I provide a first thermally-actuable control switch for said electric heater 2| mounted on the tank at the lower end thereof, and have elected to show it as comprising a tubular member with closed inner end suitably secured in a fluidtight manner to the wall of the hot water tank water and that shall be effective to delay energi-,

zation of the electric heating means with a preset relatively long time period of delay in case of withdrawal of only nominal quantities of hot water from the tank.

Another object of my invention is to provide a control system for a domestic hot water tank that shall be effective to delay energization of the electric heating means, in case-of withdrawal of nominal quantities of hot water during the early morning or forenoon hours and during the late afternoon or early evening hours, unti1 the periods of light load on the supply circuit, occurring,us-

' ually during the early afternoon and late night hours.

Another object of my invention is to provide a means to replace the time clock switch now being used in water heater control systems.

H. An expansion rod 21 is fitted into the tube 25 and normally engages against a small lug or block 29 of electric-insulating material suitably secured to the outer free end'of a resilient contact arm 3|. The contact arm 3| is adapted to engage with and be disengaged from a relatively rigid contact arm 33, and I have shown the support for the two contact arms 3i and 33 as constituted by a block 35 of electric-insulating material. It is to be understood that the showing of this heater control switch is schematic only and that I may use any structure operative for the same purpose. 1

At about the middle portion of the tank I mount a thermal retarder heater control switch unit which is disclosed and claimed'in my copending application, Serial No. 511,387, filed November 22, 1943, andv assigned to the same assuch description thereof will be here given as'is Other objects ofmy invention will either be obvious from a description of a preferred. embodiment of my control system or will be specifically pointed out in the course of such description-and particularly set forth in the appended claims. p

In the drawings,

Figure 1 is a view, in vertical section, through a Fig. 2 is a diagram of electric circuits under certain operating conditions,

Fig. 3 is a graphical diagram showing current flow through the electric heating means relative to the thermal retarder period as controlled by the tank water temperature,

, hot'water tank having associated therewith the parts comprising my improved control system,

shown in-the positions occupied by them when the water at an intermediate portion of the tank is hot and at the start of a, retarder period.

Referring first of all to Figure 1 of the drawnecessary to describe the same sufiiciently to permit its operating method to be understood.

Referring to Figs. 4 and 5 I have there shown one form of thermal retarder switch unit including a support bracket 31 having an arcuate inner surface corresponding to the arcuate outer surface of tank II to which it may be secured as by welding seams 39 or by any other suitable or desired means. 'Bracket 31 is made of a metal or alloy having high heat-conducting characteristics. A bracket or support ll, of high heat-conducting characteristics has an opening 43 therein to receive the lower reduced end portion of a, first high expansion rod 45, which rod, preferably but not necessarily issupported by bracket ll subrods 45 and 56 have their lower ends held rigidly in their brackets in any suitable or desired manner. A bore 51 extends from the lower end of rod 55 up to almost its upper end and an electric heating coil 58 is.positioned within the bore 51 as by being wound on a rod SI of electric-insulating material. A casing", of heat-insulating material surrounds the rod Iito make it more or less independent thermally of the tem- I may provide heat-insulating perature of the ambient air, to permit of the use of relatively low wattage heating coil 59 and to heat insulate the other parts of the retarder switch unit therefrom. The end portions of coil 59 are connected to terminals 65 mounted on a block 61 of electric-insulating material, which ing portions are pivotally supported on arms 69 and 1|. as by short pivot trunnions'15 and 11. A spacer rod19 secures the upper parts of the vertical portions of upper toggle member 13 into proper operative positions relative to each other.

- A lower toggle member 8| has pivotal engagement of its upper end with the lower end of toggle member 13 and an over center spring 83 has its upper end connected with a cross bar 85 in the upper toggle member 13 and its lower end connected with an intermediate part of the lower toggle member 8| to cause snap acting movement of the lower toggle member, all in a manner now well known in the art. A contact bridging member 81 is insulatedly supported at the lower end portion oftoggle member 8| and is adapted to be moved into engagement with and be disengaged from a pair of fixed contact members 89 supported on and by'block 69. A stop member 9| is.

provided for toggle member 8|, the stop being in the shape of a screw 9| having screw threaded engagement with a bracket 93 and a lock nut 95 being provided on screw 9| to permit of ad-' justably holding the stop screw 9| in any desired or required position.

In the positions occupied by the parts as shown in Fig. 4 of the drawings the temperature vof the first expansion rod 45 is substantially the same as the temperature of the second expansion rod 55, both being on the order of'70 F., and therefore the lengths of the two arms will be substantially the same and the design, construction and adjustment of the parts of the toggle switch is such that the contact bridge member 87 will be in electrical engagement with the pair of contact members 99. Y

In case the first expansion rod "45, which is connected to the tank by the supporting block 31, is subject to the temperature of hot water in the tank, on the order of 150 F., its length will be reater than that of rod 55, so that the free end of arm Bland particularly the contact bridging member 8'! will be moved out of engagement with contact members 89, and arm 8|' will be in operative engagement with stop member 9|, all as shown in Fig. 5 of the drawings.

As was hereinbefdre stated the parts of my control system shown in Fig. 1 of the drawings are in the position they will occupy when all of the water in the tank is hot so that expansion rod 45 will be elongated until the. contact bridge member 81 is out of engagement with the pair of contact members 89. Further, the expansion rod 21 will also be elongated until engagement by the contactarms 3| and 33 will be broken and therefore the heating means 2| will not be energized from a supply circuit including lines 91 and 99.

means 2| is connected in series electric circuit with the lower thermally-actuable switch and particularly the contact arms 3| and 33 as well as with the contact members 89 of the thermal retarder heater control switch unit.

Referring now to Fig. 3 I have there shown a twenty-four hour period on a base line I03. In the average home or household someone, perhaps the man of 'the house, will get up at around 6:30 a. m.-and draw on the hot water tank for a small amount of hot water as shown by the first step of the 'full line curve N15. The vertical distance between the line I03 and line I05 may represent the amount of cold water in the tank and the vertical distance between the curve H15 and an upper base line llll may represent the amount of hot water still in the tank. Further withdrawals of small quantities of hot water may occur until say 9 a. m. or 10 a. m. When the amount of hot water withdrawn from the tank is sufficient to cause the tube 25 of the lower heater control switch to be subject to cold Water, it will close and thereby cause energization of the small heating coil 59 and the temperature of the second expansion rod will be raised slowly. This will continue for four or five hours when the temperature of rod 55 will have reached say 300 F. and its length will have increased to a value causing closure of the thermal retarder heater control switch at say12.:30 p. m. or so and energization of the heater 2| which energization will continue until substantially all of the water in the tank is hot, when the lower heater control switch will open and deenergize the heater. The full line ")9 may represent the current curve.

The above described operation will occur'in dependent of withdrawals of additional small quantities of hot water so long as such withdrawals do not cause the thermal retarder and particularly the first expansion rod to become subject to cold water. If, however, an abnormal amount of hot water is withdrawn from the tank, at say 9 a. m., as shown by the broken line ill so that the amount of hot water remaining in the tank is represented by the line I I3, the first expansion rod 45 will be subjected to the temperature of cold water, its length will be quickly decreased with attendant closure of the switch and immediate energization of the electric heat- Referring now to Fig. 2 of the drawings I have there shown contact arm 3| in engagement with contact arm 33 because of the fact that expansion rod 2? is shortened by reason of its being subject to the temperature of cold water in the lower end portion of the tank. The small electric heating coil 59 in the thermal retarder unit has one of its ends connected with supply circuit conductor 91 while its other end is connected .to one terminal of an adjustable rheostat IN, the other terminal of which is connected to contact arm 3| and to one terminal of heater 2|. When contact arms 3| and 33 are in engagement with each other, as shown in Fig. 2, an energizing circuit will be closed through the small heater 59 and the design, construction and adjustment of the thermal retarder unit is such that the length of time required for the heat generated in the'small heater 59v to cause sufllcient lin'ear expansion of"; the second expansion rod 55 to the extent that the contact bridge member 81 will be moved into enagement with contact members 89 is on the order of four to six hours. I have illustrated one It is here pointed out that the electric heating method of varying the time period of delay by use of an adjustable rheostat II which may be manually adjusted to obtain the desired time period of delay, although other methods and means to obtain the same result may be employed.

Energization oi? the electric heating means 2| will continue for a time period which occurs at substantially that part of the day, that is in the early afternoon, when the usual or ordinary demands for current on the supply circuits is relativeiv small or what might be termed during the day off-peak period. It is of course to be understood that the length of time of energization of the heating means 2| is in accordance with the amount of cold water in the tank and heater 2| is deenergized by action of the lower thermal switch.

At or about 5:30 to 6:00 p. m. supper is usually in course of preparation and hot water will be demanded as is shown by the stepped curve H5 in the right hand end portion of Fig. 3. When enough hot water has been withdrawn from the tank to subject the lower thermally-actuable switch to cold water, heating coil 59 is again energized; and, at the end of a pre-set adjustable time period, current will again flow through electric heating means 2| through the two contact arms II and 33 and through fixed contacts 89 and the contact bridge member 81. If it be assumed that by about 7:00 o'clock in the evening the lower thermally actuable switch was subject to cold water such energization will start at or'about mi night as is shown by the current curve I I1.

Should abnormal demands for hot water be made early in the evening so that the amount of hot water withdrawn from the tank is such that the thermal retarder switch unit is subject to cold I water at say about 9 p. m., energization of the heating means 2| will start immediately as shown by the current curve H9 in broken lines andthe amount of hot water withdrawn being, more than line III.

R is again to be understood that the, design} construction and adjustment of the various parts of the thermal retarder switch unit is such that the average home having electric water heatin and going through the usual normal routine, will cause-energization oi the electric heating means for the tank during the oft-peak periods occurrin usually during the early afternoon hours and during the late night hours and ending say at 6 a. m. The length of time of energlzation of the electric heating means is of course dependent upon the amount of cold water in the tank, but as stated above, the design, construction and adjustment oi the various parts of the system are such that the tank will be substantially filled with hot water before the usual on-peak periods. If because of an abnormal demand during the late night hours more than half 01' the tank-full oi. hot water was withdrawn, energization of the heater would continue until substantially all of the water in the tank is hot.

-half of the tank-full as shown by the dashed f periods of heavy demand and permits energizetion of the electric heating means during the oil-- peak periods and depends more'particularly upon the ordinary habits of the people as regards the use of hot water. When it is found that one family is very abnormal in its demands for hot water it is relatively easy to install say a larger tank instead of the one of more or less standard capacity so that the control system embodying my invention can be adjusted to the habits or the few families who do not follow the usual normal routine.

The control system embodying my invention thus provides a means replacing the oil-peak timer-controlled switch, the use of which is attended by seriou limitations. One of the most serious limitation of a timer controlled switch is that it stops when the supply of current is interrupted, so that in case of one long or several short interruptions, the oft-peak period provided by such a switch will not occur at the proper time. The cost of such timer-controlled switch is also much greater than that of my thermal retarder.

Changes may be made in the details and if within the scope of the claims, are to be considered as being covered thereby.

' I claim as my invention:

1. A control system for an electric heater for a hot water tank adapted to be connected to an electric supply circuit having low load demands I during the early afternoon and the late night hours, said system including a thermal retarder heater control switch unit adapted to be subject to tank water temperature intermediate the ends of the tank and having an electricheating coil,

energize said heating coil and cause closure of said thermal retarder switch with a period of time delay after start of energization of said fheating coil to bring the closure into the early exceptions in any community to the ordinary normal living routine but when a relatively large number of homes have electrically-heated hot water tanks it is obviou that slight variations of the average normal habits and routine will tend to equalize each other, and such, few cases as may occur will, as has already been hereinbefore P inted out, be unobjectionable. V

The control system embodying my invention afternoon and during the night hours.

2. A ccntrol system for an electric heater ior a hot water tank adapted to be connected to an electric supply circuit having low load demands during the early afternoon and the late night hours, said system including a thermal retarder heater control switch unit adapted to be subject to tank water temperature intermediate the ends of the tank, and having an electric heating coil, a

thermally-actuable heater control switch subject to tank water temperature at the lower end of the tank and adapted to be closed when a sufilcient quantity of hot water has been used in the morning and in the early evening to subject only said thermally-actuable switch to cold water'to energize said heating coil and cause closure of said thermal retarder switch with a period of time delay after start of energization of said heating coil to bring the closure into the early afternoon and during the night hours, said thermally-actuable switch being eflective to deenergize said heater when substantially all of the water in the tank is hot.

,3. A control system for a hot water tank having an electric heater, comprising a thermallyactuable heater control switch subject to tank water temperature at the lower portion of the thug dispenses with the need or a timing means 7| tank and a thermal retarder heater control switch unit mounted in heat-conducting relation to the tank intermediate its ends and comprising two dissim'ilarlythermally-expansible elements.

4. A control system for an electrically operated storage water heater having an electric heater, comprising a thermally-responsive heater control switch subject to tank water temperature at the trol switch subject to tank water temperature at the lower end of the tank and a thermal retarder heater control switch unit including a first thermo-sensitive element adapted to be mounted in heat-conducting relation to the tank intermediate its ends, a second thermo-sensitive element subject to heat exchange applied independent of the tank water and auxiliary means for heat exchange with said second element controlled by said thermally-responsive switch.

subject to tank water temperature at the lower end or the tank, a thermal retarder heater control switch unit for the circuit including a thermo-sensitive element adapted to be subject to heat exchange with the tank water intermedi ate the ends or the tank and heat control means operated by power from the circuit and controlled by the thermally-responsive switch for modifying the effect of the thermo-sensitive element to vary the operation of the retarder switch.

.10. A control systemror the heater or a hot water tank, including a thermally-responsive heater control switch subject to tank water temperature at the lower portion or \the tankand a thermal'retarder heater control switch unit comprising a first thermo-sensitive element adapted to be subject to heat exchange with tankwater intermediate the ends of the tank a second thermo-sensitive element subject to heat exchange applied independent of the tank water and auxiliary means for heat exchange with the second element. I

11. In an oil-peak heating system tor heating tap water during periods of each day when the demand for electric energy is reduced, comprising an-electric heater for the tank, two thermallyactuable switches jointly effective to control the energization of said electric tank heater, the "first 6. A control system for an electrically operated storage water heater having an electric heater, comprising a thermally-responsive heater control switch subject to tank water temperature at the lower end of the tank and a thermal retarder heater control switch unit including a first thermo-sensitive element adapted to b mounted in heat-conducting relation to the tank intermediate its ends, a second thermo-sensltiv'e element subject to heat exchange applied independent of the tank water and an auxiliary heater for said second element controlled by said thermally-responsive switch in accordance with the tempggiture of the water at the lower-end of the 7. A control system for. an electrically operated storage water heater having an electric heater, comprising a thermally-responsive heater con trol switch subject to tank water temperature at the lower end ofthe tank and a thermal-retarder heater control switch unit comprising two dissimilarly thermally-responsive elements, one of said elements being adapted to be mounted in heat-conducting relation to the tank intermediate the ends thereof, the other of said elements being subject to heating applied independent of tank water and a heating coil for heating the other element, the energization of said heating coil being-controlled by said thermally-responsive heater control switch.

8. A control system for an electrically operated storage water heater, comprising a heater circuit for connection to a supply line, a therm'ally-responsive heater control switch for the circuit subject to tank water temperature at the lower end of the tank, a thermal retarder heater control switch unit for the circuit including a thermo-sensitive element adapted'to be subject to heat exchange with the tank water intermediate the ends oi the tank and heat control means operated by power from the circuit for modifying the effect of the thermo-sensitive element to vary the operation of the retarder switch,

9. A control system for an electrically operated storage water heatencomprising a heater circuit for connection to a supply line, a thermally-re:

sponsive heater control switch for the circuit of said switches being adapted to be subject to tank water temperature near the lower end of the tank, said second switch being adaptedto be subject to tank water temperature intermediate the ends of the tank, said second switch being thermally controlled by said first switch to be moved into position to energize said heater dure 'ject to tank water temperature at the lower end of the tank andadaptedto be closed when sub- 'ject to cold water temperature and a thermal retarder heater control switch unit adapted to be subject to tank water temperature intermediate the ends or the tank and electrically controlled by said thermally-actuable switch, said two switches being jointly effective to eifect energiza tion of said electric heater a predetermined period of time after entry of enough cold water into the tank to reach said thermally-actuable switch,

13. A control system for an electrically operated storage tank having an electric heater, comprising a heater circuit for connection to an electric supply line having oil-peak periods in the early afternoon and during the late night hours,

a thermally-responsive heater control switch adapted to be subject to tank water temperature at the lower end portion of the tank and tobe closed when subject to cold'water, a thermal retarder heater control switch unit comprising a first thermo-sensitive element adapted for heat exchange with tank water intermediate the ends of the tank, a second-thermo-sensitive element subject to heating applied independently of tank water and heating means for said second element operated by power from the circuit and controlled by said thermally-responsive switch, said two heater control switches being jointly effective to energize said electric heater with an adjustably predetermined time delay period after subjection of only said thermally actuable switch to cold water, said time delay period being such period. p 14. A control system for an electrically operated storage tank having an electric heater,

. comprising a heater circuit for connection to an electric supply line having off-peak periods in the early afternoon and during the late night hours, a thermally-responsive heater control switch adapted to be subject to tank water temperature at the lower end portion of the tank and to be closed when subject to cold water, a

- actuable switch to cold water, said time delay period being such as to bring the energization into an oil-peak period, said therrnally-actuablo heater control switch being effective to deen ergize said electric heater when substantially all 01 the water in the tank is hot.

15. A control system for a hot water tank having an electric heater and being subject to with-= drawal of varying quantities oi hot water durin a twenty-four hour day, said heater being adapted to be'connected to an electric supply circuit having off-peak periods during early afternoon and during late night hours, said system comprising a thermal-retarderheater control switch unit in cluding a first theme-sensitive element subj ct to heat exchange with tank waiver intermediate the ends or the tank, a second theme-sensitive element cooperating with the first element to va -1V the opening and closing or the heater-control switch and operated by power from the circuit and a thermally-actuable heater control switch subject to tank water temperature at the lower end of the tank and adapted to be closed when subject to cold water and controlling the power for said second element, said two heatereontrol switches being jointly effective to cause energlza= tion of said electric heater immediately after withdrawal of enough hot water from the tanl: to cause the first element or" the thermal retarder unit'to be subject to cold water and to cause energization of said electric heater with a time period of delay after withdrawal or only enough hot water during on-peak periods to subject only said thermally-actuable switch to cold water, said tielay period being long enough to cause the energization to start' during an oil-peak period.

16. A control system for a hot water tanlr having an electric heater and being subject to withdrawal oi varying quantities oi. hot water dui'ina a twenty-four hour day, said heater being adapted to be connected to an electric supply circuit having oil-peak periods during early afternoon and during late night hours, said system comprising a thermal-retarder heater control switch unit including a first thermo-sensitive element subject to heat exchange with tank water intermediate the ends of the tank, a second thermo-sensitive element cooperating with the first element to vary the opening and closing of the heater-control switch and operated by power from the circuit and a thermally-actuable heater control switch subject to tank water. temperature at the lower 2,876,380 as to bring the energization into an off-peak end of the tank and adapted to be closed when subject to cold water and controlling the power for said second element, said two heater control switches being jointly efl'ective to cause energizetion of said electric heater immediately after withdrawal of enough not water from the tank to cause the first element or the thermal retarder unit to be subject to cold water and to cause energization of said electric heater with a time period oi? delay after withdrawal of only enough hot water during on-peak periods to subject only said thermally-actuable switchto cold water, said delay period being long enough to cause the energization to start during an off-peak period, said thermally-actuable heater control switch being effective to deenergize said electric heater when substantially all of the water in the tank is hot.

it. A control system for a hot water tank having a cold water inlet at the lower end of the tank and a hot water outlet at the upper endv of the tanl: and being subject towithdrawal" oi varying quantities of hot water during a twentyplace during the early afternoon and during the four hour day, comprising an electric heater tor the tank adapted to be connected to an electric supply circuit having off-peak; periods during early afternoon andlate night hours, a thermally-actuable heater control switch adapted to be subject to tank water temperature at the lower end of the tank and adapted to be in closed position when subject to cold water, a thermal retarder heater control switch unit including a first thermo-sensltive unit subject to heat exchange with tank water intermediate the ends oi the tents, a second thermo-sensitive element subject to heat exchange means independent oi the tank water and operated by power irom the circuit when the thermally actuable switch is closed to be closed thereby with a predetermined time delay period to cause energization of said electric heater, after withdrawal oi enough hot water during early morning and during early evening hours to subject only said thermally-actuable switch to cold water, said energization talking late night obi-peak periods respectively.

it. A control system for an electric heater a hot water tank, comprising a thermally resp on siye heater control switch subject to tank water temperature at the lower end oi the tank and unit connected in series circuit with said the ly-responsive switch and including a i sensitive element adapted to be mounted in hurtconducting relation to the tank intermediate the ends thereof, a second thermo-sensitive element subject to heat exchange applied independent oi. the tank water, a switch arm actuable by said two elements and an auxiliary electric heating coil for said second element controlled by said thermally responsive switch, said two heater con trol switches being in closed position to ellect immediate energization of said electric heater upon the filling of said tank with cold water.

19. A control system ior an electric heater for a hot water tank. subject to withdrawals of worrying quantities oi hot water during a twenty-four hour day, comprising a thew-responsive heater control switch subject to tanl-r water temperature at the lower end oi the tank and adapted to be closed when subject to cold water and a thermal-- retarder heater control switch unit connected series electric circuit with said thermaily-responsive switch and including a lush-expansion thermo-sensltive element adapted to be mounted in heat-conducting relation to the tank interm diate the ends thereof, a low-expansion thermosensitive element subject to heat exchange applied independent of the tank water, a switch ar'm actuable by said two elements and an auxiliary heating coil 101- said second element controlled by said thermally-responsive switch, said two heater control switches being moved substantially immediately to closed position to eflect energization of said heater after withdrawal of substantially all of the hot water from the tank and entry of a corresponding amount of cold water into the tank.

20. A control system for an electric heater for a hot water tank subject to withdrawals or varying quantities of hot water during a twenty-four hour day, comprising a thermally-responsive heater control switch subject to tank water temperature at the lower end of the tank and adapted to be retarder heater control switch unit connected in series electric circuit with said thermally-responsive switch and including a high-expansion thermo-sensitive element adapted to be mounted in heat-conducting relation to the tank intermediate the ends thereof, a low-expansion thermo-sensitive element subject t heat exchange applied independent of the tank water, a switch arm actuable by said two elements andv an auxiliary heating coil for said second element controlled by said thermally-responsive switch, said two heater control switches being moved substantiallyimmediately to closed position to effect energization of said heater after withdrawal of enough hot water from the tank and entry of a corresponding amount of cold water into the tank, to subject said low-expansion element of the thermal retarder switch to cold water.

21. A control system as set forth in claim 12 and including means for varying said predetermined period 01' time v 22. A- control system as set forth in claim 12 and including manually-adjustable means for varying said predetermined period of time.

23. A control system as set forth in claim 12 and closed when subject to cold water and a thermalenough cold water into the including manually-adjustable means forvarying 4 the length 01' said time'period of delay to cause energization oi said heater to start at a predetermined time of an oflpeak period.

24. A control system for an electrically operated storage water heater having an electric heater, comprising a thermally-responsive heater control switch subject to tank water 7 lower end of the tank and a thermal-retarder heater control switch unit comprising two dissimilarly thermally-responsive elements, one of said elements being adapted to be mounted in heatconducting relation to the tank intermediate the ends thereof, the other of said elements being subject to heating applied independent of tank water and a heating coil for heating the other element, theenergization of said heating coil being controlled by said thermally-responsive heater control switch, said thermal-retarder switch being effective to cause immediate energization of said heater in case of withdrawal of enough hot water from the tankto cause entry of enough cold water into the tank to subject both switches to cold water and to prevent energization of said heater until after the elapse of a predeterminated length of time after the entry of only enough cold water into the tank to subject the thermally-responsive switch to cold water.

25. A control system for a hot water tank having an electric heater, comprising a thermallyactuable heater control switch adapted to be subject to tank water temperature at the lower end of the tankand adapted to be closed when subject to cold water temperature and a thermal retarder heater control switch unit adapted to be subject to tank water temperature intermediate the ends of the tank and electrically controlled by said thermally-actuableswitch; said thermal re tarder switch being eiiective to cause immediate energization of said heater incase of withdrawal of enough hot water from the tank to cause entry 01' enough cold water into the tank to subject both switches to cold water and to prevent energization of said heater until after the elapse of a predetermined length of time after the entry of only to cold water.

temperature at the tank to subject the 

